Abstract
Preservation of adaptive variation is a top priority of many species restoration programs, but most restoration activities are conducted without direct knowledge of selection that might foster or impair adaptation and restoration goals. In this study, we quantified geographic variation in selection on fry size of endangered Atlantic salmon (Salmo salar) during the 6-week period immediately following stocking in the wild. We also used a model selection approach to assess whether habitat variables influence patterns of such selection. We found evidence for significant size-selection in five out of six selection trials. Interestingly, the strength and pattern of selection varied extensively among sites, and model selection suggested that this variation in phenotypic selection was related to geographic variation in the presence of large woody debris and the slope of the stream gradient. The strong selection differentials we observed should be a concern for endangered salmon restoration, whether they reflect natural processes and an opportunity to maintain adaptation, or an indicator of the potentially deleterious phenotypic consequences of hatchery practices.
Highlights
Size at age is considered a critical adaptive trait for most organisms
Study system In 2000, the Gulf of Maine Distinct Population Segment (GOMDPS) of Atlantic salmon was listed under the US Endangered Species Act (ESA) due to dangerously reduced spawning runs and low juvenile densities [National Research Council (NRC) 2004]
The six sites in 2nd and 3rd order streams were distributed among five river systems: mainstem of the Dennys River (DEN), mainstem of the East Machias (EMA), Mopang Stream in the Machias River (MOP), Shorey Brook in the Narraguagus River (SHO), a mainstem site (SMA) in the Sheepscott River, and a site in the West Branch of the Sheepscott (SWB)
Summary
Larger size at age is thought to increase predator escape ability, environmental tolerances, and competitive ability, promote earlier maturation, and contribute to greater fecundity (reviewed in Sogard 1997). Optimal size at age presumably varies in space and time depending on a number of costs and benefits specific to local environmental conditions. Understanding how selection for size at age varies geographically and with habitat features is important in understanding the origins and maintenance of size and traits linked to size. Perhaps nowhere is such adaptive insight more practically significant than for endangered or threatened species. We show that such a ‘selectional’ assessment is both feasible and informative
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